A field experiment was conducted in a re-established g asslandin northern Utah, USA, to examine whether grasshopper species interact strongly when natural populations reach high densities. During the growing season of 1990, plots with average densities of 10-11 grasshoppers per m2 were treated with carbaryl bran bait (carbaryl plots) or bran bait without carbaryl (control plots). The grasshoppers fed particularly heavily on alfalfa, driving the percent cover of this species to essentially zero by mid-August. Application of carbaryl significantly reduced the densities of three of the four major grasshopper species present (Oedaleonotus enigma, Melanoplus sanguinipes, and M. packardii), and slowed the rate with which these insects consumed the standing crop of alfalfa. In contrast, densities of the fourth major grasshoppers pecies present, M. bivittatus, were not reduced by application of carbaryl but instead were greatly elevated. This population response may have arisen in large part as adults of M. bivittatus aggregated in carbaryl plots in response to the reduced rate of defoliation of alfalfa by other species. Greater numbers of M. bivittatus nymphs also occurred in 1991 in plots treated with versus without carbaryl in 1990 (no carbaryl was applied in 1991), as did greater numbers of O. enigma nymphs (despite there having been fewer adult females present in these plots the previous year). But the densities of grasshoppers of all species were low in all plots in 1991 (approximately 1 perm 2 by mid-summer), and no differences in percent cover of alfalfa or other vegetation were detected in plots treated the previous year with versus without carbaryl. The striking positive response of M. bivittatus to application of carbarybl ran (and associated reductions in population sizes of competing grasshopper species) strongly suggests that intense interspecific interactions occurred at the study site in 1990.